The present invention relates to enveloped, negative-sense, single-stranded RNA viruses and to the use of such viruses as live attenuated vaccines. Specifically, the invention relates to new human vaccines for enveloped viruses such as parainfluenza, respiratory syncytial virus, measles and influenza viruses, among others. The invention also relates to a method for screening such vaccines to ensure attenuation prior to their administration and to check the stability of the attenuated strain after administration.
A number of viruses may cause severe infections in humans and animals. For example, respiratory syncytial virus (RSV) and parainfluenza virus are two of the leading causes of severe upper and/or lower respiratory tract disease in neonates and young infants. Other viruses, such as influenza virus, measles virus and human immunodeficiency virus, are also of significant concern.
A variety of vaccines have been developed over the years to prevent viral infections in animals and humans. Two principle types of vaccines have been used: killed viruses and attenuated live virus. A killed virus is typically inactivated by chemical or physical treatment, but is generally less effective in stimulating a lasting immune response than an attenuated live virus. Attenuated live viruses are typically more effective, but may revert back to their virulent state while in the body. The time and cost involved in developing either killed or live vaccines is significant.
Live, attenuated vaccines may be obtained directly from progeny viruses isolated from infected animals. For example, U.S. Pat. No. 3,927,209 to Straub discloses a parainfluenza type-3 vaccine isolated as a virus strain from a bovine respiratory tract. Live attenuated vaccines may also be obtained by repeatedly cold passaging a wild-type strain through suitable cultures until the virus has lost its original pathogenic properties. For example, cp45, a cold-adapted, temperature sensitive strain was obtained by passing the wild-type virus (JS strain) of HPIV-3 45 times at reduced temperatures. (Belshe and Hissom, 1982). The temperature sensitive cp45 strain is currently under evaluation for use as a candidate vaccine in humans. (Karron et al. 1995; Hall et al. 1993; Belshe et al. 1992; Clements et al. 1991; Crookshanks-Newman and Belshe 1986). Recent evaluation in children has revealed the cp45 strain to be highly attenuated and effective in stimulating immunogenic response. (Karron et al. 1995; Belshe et al. 1992).
Attenuation in a particular vaccine strain is commonly evaluated with respect to three phenotypes of the strain: cold adaptation, temperature sensitivity and plaque size or yield in tissue culture. Cold adaptation relates to the ability of the virus to grow at 20.degree. C. and the temperature sensitivity relates to whether such growth is inhibited at temperatures of around 40.degree. C. Plaque titers are an assay for quantitatively evaluating the extent of virus growth, and are commonly used to evaluate the extent of cold-adaptive and/or temperature sensitive phenotypes. Other methods for determining whether an vaccine is attenuated involve administering the vaccine to primates. For example, new polio vaccine lots are typically administered to monkeys before being approved for sale by the FDA.
A continuing need exists for developing new vaccines. The prior art methods of developing live attenuated vaccines by cold passaging, while often effective, are not predictable as to their success, and are necessarily limited to application against a single virus. A need also exists for alternative methods to determine whether a virus is sufficiently attenuated. Characterization of cold adaptive and temperature sensitive phenotypes are not definitive. Administration of vaccines to test animals are likewise not definitive, and are inefficient.